As a preparation for soldering oxide materials such as ceramics and glass, a process of applying an electroplating or electroless plating, such as gold plating, copper plating, and nickel plating, on the oxide materials is widely known. However, it is expensive and complicated to solder plated surfaces. Therefore, it is desired to develop an economic solder alloy.
To comply with the aforementioned demand, Japanese patent publication 49-22299B and Japanese patent publication 52-21980B disclose Pb--Sn solder alloys which can be directly soldered to glass and ceramics.
However, lead is toxic and has been recognized as hazards to injure human health and to hurt the environment, affecting the ecosystem, so the use of lead poses a problem. The trend to use no lead in solder alloy has been rapidly increased.
The solder alloy disclosed in the above Japanese patent publication 49-22299B is a Pb--Sn--Cd--Sb solder alloy which can be directly soldered to an oxide film material such as glass and ceramics, but includes toxic lead. The lead elutes from the abandoned products using the solder alloy to cause serious problem to the environment when the products are exposed to acid rain.
The solder alloy disclosed in the above Japanese patent publication 52-21980B is a solder alloy containing rare earth metals which is useful for bonding oxide materials such as glass and ceramics. However, the solder alloy has the same problems as above because it contains lead as a main component.
Development has been actively carried out to provide improved lead-free solder alloys especially for mounting electronic components to a printed wiring board. For example, an Sn--Ag--In solder alloy is disclosed in Japanese patent publication 9-326554A, and an Sn--Zn--Bi solder alloy is disclosed in Japanese patent publication 8-164495A. However, their bonding strengths are not enough for oxide materials such as glass and ceramics.
As an example of lead-free solder alloys for soldering metal oxide materials, an Sn--Ag--Al--Zn solder alloy is disclosed in Japanese patent publication 55-36032B. The resultant solder layer easily separates from oxide material such as glass and ceramics because this solder alloy is for soldering metal and therefore the coefficient of thermal expansion of the solder alloy is greatly different from that of the oxide material.